Power electronics transistors are widely used in a variety of applications such as mobile phone network, solar farms and electrical cars to modify the waveform of electricity, for example, from dc (direct current) to ac (alternating current) or from one voltage level to another. These transistors need to operate over a wide range of power levels, from milliwatts in mobile handsets to hundreds of megawatts in a high-voltage power transmission system, and at different frequencies for a radio frequency to microwave and mm-waves. In spite of the progress made in the production of these transistors with a handful of materials such as Si, SiC, arsenide and III-nitrides (GaN, AlN, InGaN, AlGaN etc), the devices have pros and cons. Particularly, the available devices lack the abilities of operating at a high-voltage (>1 KV) and a high energy density with a low loss.
Particularly, the 21st century has brought challenges to the human race in decarbonising power grids that consume fossil fuels or radioactive materials. To date, the mass production of purified Si material and silicon solar cells has significantly reduced the costs of solar panels, with solar farms supplying energy to global power grids awaiting on the horizon. The dc power generated by solar farms must be changed efficiently into 3-phase ac with electronic circuits, known as invertors, or the voltage has to be increased to up to 1200V with convertors to reduce the current flow so that Joule heating can be minimised. In recent years, similar invertors and convertors also have been sought urgently for the regulation of power in electric vehicles (EV) and solar airplanes. These convert the low DC voltage of batteries (<40 V) to more than 1 KV, or invert a DC voltage to 3-phase AC to drive an electrical motor. For electric vehicles, these circuits can account for a quarter of all costs.
The efficiency of these convertors or invertors is limited by the transistors that act as switches. These transistors have to meet these requirements: 1) low energy loss that makes solar farms more cost-effective or enables cars or solar planes to travel for longer distances; 2) high breakdown voltage; 3) robustness and thermal stability in harsh operating environments; 4) built-in mechanisms for heat dissipation.
However, these transistors available in the market have never been ideal. 1) Si insulated gate bipolar transistors (IGBTs) have been widely used in inverter/converter modules. However, a high percentage of energy loss (>15%), a low breakdown voltage and a low switching rate are the major disadvantages. 2) Transistors made of wide-bandgap SiC (band gap: 3.26 eV) are the state of the arts to date. But the growth technology for SiC has never been mature and the wafer cost is expected to be at the same level (typically ˜400 USD/wafer, 2″; or 2000 USD/wafer, 4″)/ Also, the wafers contain defects such as micro-pipes and hollow tubes that are regarded as the killer defects for SiC high-power and high-voltage transistors.
III-nitrides such as AlN and GaN have properties similar to those of SiC, which include wide band-gaps, good thermal conductivities and high electrical strengths. But they are different from SiC. They are polar materials and have spontaneous polarizations along one direction known as C axis. So, for a thin piece of III-nitride having C-axis as its surface normal, its top C-plane and bottom C-plane are not equivalent. Technically, one is referred as III-polar face (hereafter III-face or (0001) plane, such as Ga-face for GaN) and the other is referred as nitrogen-polar face ((000-1) plane or N-face). But when C-axis is parallel to a thin piece of wafer, the wafer is known as non-polar III-nitride and hence C-plane, III-face and N-face are parallel to the surface normal.
At a hetero-junction of two different C-plane III-nitrides such as Al0.25Ga0.75N/GaN and In0.36Al0.64N/GaN, there are discontinuities in energy band and polarization. These discontinuities can lead to both a charge transport channel of a high electron mobility and a two-dimensional electron gas (2DEG) that can be modulated by an electrical field. So, they have been employed for forming field-effect transistors (FET), which is also known as high-electron mobility transistors (HEMTs) that operate at a current density of ˜1 A/mm. But the operating voltage of these transistors is in general low (<700 V). So attempts have been made to increase the breakdown with vertical structure designs.
Okada et al demonstrated a vertical structure with a conducting 2DEG channel on a tilted C-plane on a Ga-polar GaN substrate. But the conductive channel contains steps that are formed on a tilted C-plane during growth. So, the electron mobility of the channel is reduced. Therefore, a high resistance is reported. Also, a current aperture vertical electron transistor (CAVET) has been proposed and demonstrated by Ben-Yaacov et al and later by Kanechika et al with a horizontal 2DEG forming a part of the channel. The source region consists of an AlGaN/GaN hetero-junction (i.e. 2DEG), and is separated vertically from the drain region by an insulation III-nitride layer. The insulation layer contains an vertical aperture that is filled with a conductive material such as Si:GaN, the same as the drain region. Therefore, electrons can flow from the source along the 2DEG, through the aperture, and collected at the drain. A gate, that is located directly above the aperture and larger than the aperture, is used to modulate the charge in the 2DEG conducting channel to control the amount of current that passes through the aperture. But these transistors often exhibit internal current leaks that cause device breakdown since the aperture is formed on ICP (inductive coupled plasma) etching surfaces. Further, it is obvious that such a 2DEG can be arranged vertically on the III-face or N-faces of a non-polar template (i.e. C direction is horizontal), as proposed by Khalil et al (Pub. NO.: US 2015/0014700 A1), solely as a conductive channel from the gate to the drain.
There are also designs that employ the high electron mobility of bulk nitride materials. These include GaN junction field effect transistors (JFET) and vertical GaN trench metaloxide-semiconductor field-effect transistor (MOSFET). These devices have a high operating voltage that benefits from the vertical design. But, the electron mobility is reduced to the electron mobility of bulk GaN and as a result, the device resistance is increased. Hence, a significant energy loss is expected. However, the high electron mobility of InGaN has yet to be exploited.
But there are plenty of issues with those prior arts:
First of all, the formation of these transistors often involves MOVPE (Metal Organic Vapour Phase Epitaxy) or MBE (Molecular Beam Epitaxy) growth of III-nitride on plasma- or chemically-etched surfaces. Consequently, the resulting devices will have a high density of defects that reduce electron mobility and increase current leak.
Secondly, GaN and AlN wafers are really rare and expensive (˜500 USD/cm2), and they cannot meet the demand of the industry. So, low-cost templates such as GaN/sapphire or GaN/SiC have to be used. However, these templates often have a high level of defects that must be avoided in forming high-quality devices.
Thirdly, the existing device designs involve the formation of a current pass connection from a highly-conductive, narrow 2DEG conducting channel (a few nanometer in width) to a less-conductive n-type Si:GaN bulk body. Consequently, “hotspots” are created during operation, which could cause device breakdown.
Finally and importantly, for FETs with 2DEG, in an off-state, there is a huge electrical field formed at the gate edge of drain side, which induces device breakdown through surface defects. Such an electrical field must be shifted to a region where dielectric strength is high.
Epitaxial lateral overgrowth (ELOG) (in some literature, it is known as lateral epitaxial overgrowth (LEO)) has been well-known for the reduction of defects in III-nitrides. It involves the use of a mask layer such as SO2 or SixN as growth mask. Such a layer is patterned to form opening windows to expose the surface of a III-nitride template. So, during growth, III-nitride first grows inside the opening windows, then grows not only vertically, but also laterally overlaying the growth mask. Since the mask blocks the passes of defects, the III-nitride growing over a mask has a much reduced defect density. This technology has been successfully used in Blu-Ray laser diodes. However, it has not been used for the formation of high-quality transistor because a useful transistor comprises regions of different electrical properties. For example, the drain and the source of a transistor must be conductive; the material between them must be very resistive and another material that is used to connect them must have high electron mobility. Therefore, it requires a tailored growth process to avoid “cross-deposition”, i.e. resistive material should not be inserted into a current pass or conductive material cannot be a part of an insulation layer. The former will increase the serial resistance locally to result in a hot spot that could cause transistor breakdown and the latter will cause a leakage current, i.e. a transistor cannot be turned off. Further, since ELOG growth can only offer a small quantity of materials, incorporating them into a transistor requires a thorough consideration.
Therefore, it would be desirable to be able to address at least some of the above difficulties through comprehensive structure design in accordance with growth and manufacture procedures.